Agonism is a broad term which encompasses many behaviours that result from, or are triggered by biological conflict between competing organisms. It is defined as "survivalist animal behaviour that includes aggression, defense, and avoidance". Approximately 23 shark species are capable of producing such displays when threatened by intraspecific or interspecific competitors, as an evolutionary strategy to avoid unnecessary combat. The behavioural, postural, social and kinetic elements which comprise this complex, ritualized display can be easily distinguished from normal, or non-display behaviour, considered typical of that species' life history. The display itself confers pertinent information to the foe regarding the displayer's physical fitness, body size, inborn biological weaponry, confidence and determination to fight. This behaviour is advantageous because it is much less biologically taxing for an individual to display its intention to fight than the injuries it would sustain during conflict. Agonistic displays are essential to the social dynamics of many biological taxa, extending far beyond sharks.
Characteristics
Definition
Agonistic displays are ritualized sequences of actions, produced by animals belonging to almost all biological taxa, in response to conflict with other organisms. If challenged or threatened, animals may employ a suite of adaptive behaviours, which are used to reinforce the chances of their own survival. Behaviours which arise from agonistic conflict include:
fight or flight response threat display to warn competitors and signal honest intentions defence behaviour simulated paralysis avoidance behaviour withdrawal settling behaviour. Each of these listed strategies constitute some manifestation of agonistic behaviour, and have been observed in numerous shark species, among many higher taxa in Kingdom Animalia. Displays of this nature are influenced and reinforced by natural selection, as an optimal strategy for avoiding physical conflict, and the costs of such interactions. In nature, agonistic behaviour is both species and situation-specific, and is often expressed as a graded response to imposed threats.
Triggers The most effective triggers of agonistic behaviour in sharks include:
hunger crowding by human divers (independent or group dives) and submersible machinery sustained targeted pursuit invasion of the shark's idiosphere, without appropriate warning or consent natural competition for resources with other organisms obstructions on the benthic floor which block escape routes or reduce scope of visibility crowding by boats. The intensity and duration of the resulting display graded in proportion to the shark's perceived degree of endangerment during the conflict.
Physical characteristics Apart from species which incorporate unique elements into their display, a typical threat display will include the following four postural elements: elevated snout, pectoral fin depression, arching of the back, and lateral flexing of the body to display total size, and two locomotory elements: jerky side-to-side movements and rolling or spiral looping in the water column. There are always exceptions to this rule. Certain species will display unique visual cues, outside of the displays discussed here, as individual species face their own biological stresses that drive the selection and reinforcement of new adaptive behaviours. Outside the typical display, certain species have been reported displaying the following behaviours as part of a species-specific display:
Regardless of the precise sequence, agonistic displays function as accurate, predictive warnings to competitors which communicate a willingness to engage in more intense behaviour, if further provoked. Displays of this nature will always either result in a fleeing behaviour, or rapid, slashing attack from the displaying individual.
Characteristics of normal behaviour To recognize and identify true agonistic behaviour, it is first important to understand what constitutes normal behaviour in sharks, specific to the lifestyle of the species of interest. For sharks, normal behaviour is considerably different than the exaggerated appearance of the agonistic display. A shark which is in distress will behave in a manner that is easily recognizable, due to the sheer oddity of the sequence of movements and torsional elements characteristic of agonistic displays - juxtaposed to normal behaviour, which appears visibly more relaxed and natural. The following table summarizes the characteristic physical elements of sharks displaying agonistic behaviour and non-displaying sharks:
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